viral genetics
TRANSCRIPT
1
VIRAL GENETICS
• PATHOGENESIS
• LIFE CYCLES• VACCINE DEVELOPMENT
• DRUG RESISTANCE
www.freelivedoctor.com
2
VIRAL GENETICS
“DNA chromosomes of eukaryotic host organisms generally require geologic time
spans to evolve to the degree that their RNA viruses can achieve in a single human
generation.”
www.freelivedoctor.com
3
VIRAL GENETICS
• VIRUSES GROW RAPIDLY
• A SINGLE PARTICLE PRODUCES A LOT OF PROGENY
• DNA VIRUSES SEEM TO HAVE ACCESS TO PROOF READING, RNA VIRUSES DO NOT SEEM TO
www.freelivedoctor.com
4
NATURE OF GENOMES
• RNA or DNA
• SEGMENTED OR NON-SEGMENTED
www.freelivedoctor.com
5
GENETIC CHANGE
• MUTATION
• RECOMBINATION
www.freelivedoctor.com
6
ORIGIN OF MUTATIONS
• SPONTANEOUS– tautomeric form of bases– polymerase errors
www.freelivedoctor.com
7
Tautomeric forms of bases
most of time rarely
www.freelivedoctor.com
8
ORIGIN OF MUTATIONS
• SPONTANEOUS– tautomeric form of bases– polymerase errors
why do some viruses seem to alter very little, even though one would expect high mutation rates?
mutation rates usually higher in RNA viruses (lack of proof reading)
www.freelivedoctor.com
9www.freelivedoctor.com
10
ORIGIN OF MUTATIONS
• SPONTANEOUS• PHYSICALLY INDUCED
–UV light , especially problem if no access to repair
–X-rays • CHEMICALLY INDUCED
www.freelivedoctor.com
11
TYPES OF MUTATION
• POINT
•INSERTION
•DELETION
www.freelivedoctor.com
12
PHENOTYPES
PHENOTYPE
– the observed properties of an organism
www.freelivedoctor.com
13
PHENOTYPIC CHANGES
• CONDITIONAL LETHAL - multiply under some conditions but not others - wild-type (wt) grows under both sets of conditions
• temperature-sensitive (ts) mutants do not grow at higher temperature (altered protein)
• host-range mutants do not grow in all the cell types that the wt does
www.freelivedoctor.com
14
PHENOTYPIC CHANGES
• PLAQUE SIZE– may show altered pathogenicity
• DRUG RESISTANCE– important in the development of antiviral agents
• ENZYME-DEFICIENT MUTANTS– some genes can be ‘optional’ in certain
circumstances
www.freelivedoctor.com
15
PHENOTYPIC CHANGES
• “HOT MUTANTS”– grow better at elevated temperature than wt– less susceptible to host fever response
• ATTENUATED MUTANTS– milder (or no) symptoms– vaccine development– pathogenesis
www.freelivedoctor.com
16
GENETIC CHANGE
• MUTATION
• RECOMBINATION
www.freelivedoctor.com
17
RECOMBINATION
Exchange of information between two genomes
www.freelivedoctor.com
18
RECOMBINATION‘classic’ recombination common in DNA viruses
www.freelivedoctor.com
19
COPY CHOICE RECOMBINATION
template switch
www.freelivedoctor.com
20
COPY CHOICE RECOMBINATION
continues copying
www.freelivedoctor.com
21
COPY CHOICE RECOMBINATION
www.freelivedoctor.com
22
COPY CHOICE RECOMBINATION
www.freelivedoctor.com
23
Other methods recombination
• Take advantage quirks in virus replication
– eg. Coronaviruses (include SARS virus)
www.freelivedoctor.com
24
RECOMBINATION - SOME USES
• mapping by recombination frequency
• mapping by marker rescue
www.freelivedoctor.com
25
RECOMBINATION - SOME USES marker rescue
www.freelivedoctor.com
26
RECOMBINATION - SOME USES
• mapping by recombination frequency
• mapping by marker rescue• development of recombinant viruses for
vaccines and therapeutic reasons
www.freelivedoctor.com
27
RECOMBINATION - SOME USES
www.freelivedoctor.com
28
raccoon eating bait with rabies vaccine in itwww.freelivedoctor.com
29
REASSORTMENT
www.freelivedoctor.com
30
REASSORTMENT
• form of recombination (non classical)
• very efficient• segmented viruses only
– can occur naturally
• used in some new vaccines– eg for influenza and rotaviruses
www.freelivedoctor.com
31adapted fromTreanor JJ Infect. Med. 15:714
• cold adapted
• temperature-sensitive
• attenuated
• live vaccine
• intranasal delivery
• approved 2003
INFLUENZA VIRUS
www.freelivedoctor.com
32
NON-SEGMENTED NEGATIVE STRAND RNA VIRUSES
• no classical recombination
• no copy choice• no reassortment
– least ability to exchange genetic material
www.freelivedoctor.com
33
other aspects of viral genetics
www.freelivedoctor.com
34
COMPLEMENTATIONInteraction at the functional level, NOT the nucleic
acid level
mutants which can complement are generally in different genes
Progeny virus assembled using wt N and wt M proteins
Genomes in progeny are either ts M or ts N
ts mutant 1 ts mutant 2
www.freelivedoctor.com
35
DEFECTIVE VIRUSES• lack gene(s) necessary for a complete
infectious cycle
• ‘helper’ virus provides missing functions
package me! copy me!
package me! copy me!
www.freelivedoctor.com
36
DEFECTIVE VIRUSES
• some examples of defective viruses
– some retroviruses (use related helper)
– hepatitis delta virus (uses unrelated helper)
www.freelivedoctor.com
37
DEFECTIVE INTERFERING (DI) VIRUSES (PARTICLES)
• decrease replication of helper virus
– compete for viral precursors, etc.
• may modulate wt infections
• occur naturally eg. DI measles virus in
subacute scelerosing panencephalitis -
SSPEwww.freelivedoctor.com
38
PHENOTYPIC MIXING
no changes in genome
possibly altered host range
possibly resistant to antibody neutralizationwww.freelivedoctor.com
39
PHENOTYPIC MIXING
www.freelivedoctor.com